A superabsorbent polymer (SAP) absorbs large quantities of water as well as other fluids. SAP is typically designed to be resistant to humidity, but will swell when put in intimate contact with water. SAPs are usually prepared by either one of two methods. The first method involves sufficiently crosslinking emulsion or aqueous solution polymers to make them water insoluble, while retaining their ability to swell in water. The second method is directed at effectively modifying water-insoluble polymers with pendant hydrophilic groups to induce swelling when in contact with water.
SAPs find application in the medical, food and agricultural industries. They also find utility in many consumer products, in particular disposable absorbent articles such as disposable diapers, incontinent pads and feminine care products. The ability to provide thinner, more compact absorbent articles has been contingent on the ability to develop relatively thin absorbent cores that can acquire, distribute and store large quantities of fluid, particularly urine. As a result, there is a trend towards employing higher concentrations of SAP to achieve this goal.
SAPs are available in a particulate or powder form. In the case of diaper construction, SAPs are sifted into the absorbent core. The absorbent core is sandwiched between a fluid pervious topsheet and a fluid impervious backsheet. The incorporation of particulate SAP tends to generate dust from the SAP fines. Further, conventional absorbent articles have the limitation of the SAP not being sufficiently immobilized and are thus free to migrate and shift during the manufacturing process, shipping/handling and/or use. Movement of the SAP particles during manufacture can lead to handling losses as well as improper distribution of the particles.
Further, absorbency problems occur when the SAP particles migrate prior to, during and after swelling. This inability to fix the particles at optimum locations leads to insufficient fluid storage in one area and over-capacity in other areas. Another important factor is the liquid permeability of the SAP. It has been discovered that the fluid transport properties of the gel layer formed as a result of the swelling SAP particles in the presence of fluids is extremely important. Although the formation of a SAP gel layer fluid barrier, known as “gel blocking” is desirable for some applications, such as for use in cables, the formation of gel layers in disposable absorbent products is undesirable since it greatly reduces the efficiency of the SAP. Thus, the advantages of being able to fixate SAP particles in place are apparent and several ways of accomplishing such have been suggested.
There are many patents pertaining to crosslinkable water soluble/swellable polyacrylate based compositions. However, consistent with the fact that commercially available SAP tends to be in granular, particulate or powdered form, the impetus of the prior art is aimed at making highly viscous emulsions and dispersions that are subsequently dried, masticated, pulverized or ground to the desired size.
For example, Chang et al., U.S. Pat. No. 4,914,170 issued Apr. 3, 1990 relates to superabsorbent polymeric compositions prepared from a monomer including acrylic acid and an effective amount of a second hydrophilic monomer, which can be a soluble salt of beta-acryloxypropionic acid. Preferably, the pH of the aqueous monomer solution is adjusted to substantially exclude free acid, and the aqueous monomer solution is coated onto a heated surface to both polymerize the monomer and dry the resulting hydrogel. Free acid thermal degradation products are avoided and the superabsorbent product has greater absorption capacity than acrylate homopolymer.
Example 1, column 13, describes the preparation of superabsorbent powder. Specifically, 700 g of acrylic acid and 77.8 g of distilled beta-acryoxypropionic acid and 1060.5 g of DI water are cooled to 5° C. The solution was neutralized with 725 g of 50% reagent grade NaOH. The temperature of the solution was maintained below 35° C. and additional NaOH solution was added to reach a pH of 8.0. The final solution had 36% solids. A portion of monomer solution was charged with crosslinking agent, initiator and surfactant just prior to polymerization. The reaction mixture was placed in a shallow tray providing a liquid layer with a thickness of 75 mils. The tray was placed in an oven at 175° C., covered with aluminum foil initially for a few minutes while the polymerization began. Thereafter the foil was removed and the material was allowed to polymerize for 30 to 45 minutes. The resulting polymer had less than 20% w/w water content and was ground to a powder.
As industry recognized the deficiencies of particulate SAP, aqueous based superabsorbent polymer compositions began to be developed. U.S. Pat. No. 5,693,707 issued Dec. 2, 1997 Cheng et al., teaches an aqueous polymer composition comprising 10 to 40% of a polymer in water, the polymer consisting essentially of 20-90 wt-% alpha, beta-ethylenically unsaturated carboxylic acid monomer, 10-80 wt-% of one or more softening monomers, the aqueous composition being adjusted to pH 4-6 with alkali metal hydroxide or alkaline earth metal hydroxide and further containing 0.1 to 3 wt-% zirconium crosslinking salt.